Instead of generating a suitable image header with linker magic, move
all of this into zynq_mkimage. The configuration file format and parsing
is based on imx-image. This gets us one step further on the road to
proper multi-image support.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>